Coordinate measuring machines are used for dimensional inspection of workpieces such as machine parts. A workpiece is secured to a table, and a measuring probe is secured to a movable ram. In order to measure the position of a point on the workpiece, the probe is brought into contact with the point, and the x, y and z measuring scales of the machine are read. The table for support of the workpiece is conventionally a granite block because of the stability and resistance to damage of granite. The remainder of the machine frame is typically a metal such as aluminum or steel.
One of the difficulties with a granite worktable supported by a metal base is the different temperature coefficients of granite and metal. When the coordinate measuring system undergoes temperature variations, the granite and metal elements expand or contract by different amounts, causing distortion of rigidly-connected system elements. These distortions cause errors in the measurements made with the machine and are highly undesirable. Furthermore, the granite block typically has a very long thermal time constant and takes a long time to stabilize dimensionally after a temperature change. Thus, distortions may continue over a long period of time.
A further difficulty with a granite worktable supported by a metal base is that the worktable can be deflected by the weight of the workpiece. Such deflections can be transmitted to the machine frame and cause measurement errors.
It is desirable to provide a base assembly for a coordinate measuring machine which utilizes a granite block of relatively small size to support a workpiece. The granite block must be supported by the machine base and must be stabilized against lateral and rotational movement relative to the base. Distortions caused by temperature variations, workpiece weight and other factors must be minimized. The cost and weight of the base assembly must also be minimized.
It is a general object of the present invention to provide an improved base assembly for a machine tool.
It is another object of the present invention to provide a base assembly wherein a granite block is supported by a metal base member, and support means for preventing distortion caused by differential expansion and contraction are provided between the base member and the granite block.
It is yet another object of the present invention to provide a base assembly for a coordinate measuring machine which is relatively light in weight and low in cost.
It is still another object of the present invention to provide a base assembly for a coordinate measuring machine wherein measurement errors caused by differential thermal expansion and contraction are limited.
It is a further object of the present invention to provide a base assembly for a coordinate measuring machine wherein a metal base member is isolated from worktable deflections caused by workpiece weight.
It is another object of the present invention to provide a base assembly for a coordinate measuring machine wherein a granite block is supported by a metal base member and is restrained against relative lateral and rotational movement.